Many uses of metals require high purity. This is particularly true when metallic nickel is used in the production of superalloys. For example, sulfur in amounts greater than 50 parts per million can induce hot shortness in super alloys during hot working. It has also been established that the metalloid elements including, but not restricted to bismuth, lead, tellurium and to a lesser extent selenium, lower the ductility of super alloys in short time, elevated temperature tensile tests and in creep-rupture tests. Metallic nickel can be produced by electrorefining, electrowinning, carbonyl techniques and precipitating metallic nickel with a reducing gas from aqueous solutions. The first three of these processes generally produce a refined nickel product that contains less than 10 ppm selenium. Metallic nickel precipitated from aqueous solutions contains higher selenium concentration. It is advantageous to precipitate from aqueous solutions nickel having selenium contents comparable or lower than nickel produced by the other processes.
Some selenium is removed by partial volatilization during intermediate pyrometallurgical treatments. Selenium is also partially removed from aqueous solutions by coprecipitation with iron hydroxide, e.g., when iron is hydrolyzed under oxidizing conditions at temperatures in excess of 200.degree. C. or when iron is hydrolyzed from solutions having a pH range between about 4 and 6. Some tetravelent selenium is further removed by cementation with metallic copper at moderate temperatures and with metallic nickel at temperatures above about 100.degree. C.
These known processes for removing selenium from nickel solutions are costly, only partially effective or inapplicable. For example, those processes that require the use of temperatures of above 100.degree. C. are frequently commercially unattractive because such reactions require the use of pressurized vessels in a large production stream, can entail the loss of nickel from solution or are effective in removing only selenium that is present in the tetravalent state. These known processes which rely on the coprecipitation of selenium with hydrolyzed ferric hydroxide or other hydroxides are not applicable to the treatment of pure nickel solutions because either these solutions do not contain the coprecipitating compound or are effective in only removing tetravalent selenium.
Recently, a process was described in U.S. Pat. No. 4,214,900 for removing selenium from solution by treating the solution with a semi-metallic compound containing a chromous compound. This process is effective in lowering the selenium content in solution, but the production of the semi-metallic compound is expensive.
A process for removing both tetravalent and hexavalent selenium from metal solutions under moderate operating conditions has now been discovered.